This is my first guide here, so please tell me if Im doing something wrong.

When I started playing LoL I actually had no idea what I was doing, and I used to only play bot games like most of you did, kinda funny because that's when I started to play Lux.

So I have mained Lux ever since I started playing this game, so I know a thing or two.
I find her a very strong mid if played right despite that a lot of people underrate her, if you take advantage of your massive range you can easily own the lane.

So for the runes, its quite simple:
I like to have full AP on everything to maximize your potential.
I believe this is the way to go as opposed to getting a bit of everything because Lux is just one of those champions that you want to be able to burst down your enemy faster then they can do it to you as you really don't have much escape if you don't kill them there.
You could obviously chose to do these differently, but I find this is the most effective.
If you do chose to do something else, I highly recommend getting slight attack speed to really make your passive useful.

The masteries are pretty normal, although you might notice that I took attack speed and I will explain that in a second but first it is important to know that the mana regen masteries is very important because if you spam too much you won't have enough mana to do much. Now to talk about that attack speed, I mentioned earlier that if you get some attack speed(not too much) it can me helpful to farm early on and know that out farming in mid is very important.

In this part I will explain my item choices:

Rabadon's Deathcap This is quite obvious really, but if you don't know, it gives you 140 AP and 25% AP bonus.

Void Staff This should be a MUST HAVE for Lux, it gives you the ability to go right through the enemy magic resist.

Lich Bane Once again, you should always try to get this, it is VERY useful and by the end of this build it will provide you with around 528 bonus damage on your basic attack(plus your passive) that will provide total destruction.

Deathfire Grasp You don't actually have to buy this, I put this here because it is REALLY strong for killing enemy tanks. If they don't have a tank you can buy something like an Archangle.

Morello's Evil Tome This is just a fun item to get if you have the money left, it can let you shoot your ult in what seems like seconds(it would actually be more like 24-29 second cooldown I do believe).

My skill sequence is quite easy to follow, you want to have your E as high as it can, so you can spam it some more! But you really do want to get your E first to allow for easy harass, the only real reason to EVER get Q first if your team is going to invade or something(either that or you are supporting). You will find that your E wont do much at the start, but if you use it really often you can lower them just enough to go into range, ignite and use R then BOOM :P

Ignite This is nice to use because you can use ignite to make sure you get that kill(nothing is more annoying then using your ult then having them run away with like 20 health)

Flash You should have this at all times because its really useful to have when you want to escape as you literally have NOTHING to get away other then trying to throw your CC backwards.

There are a few pros and cons on this build:
Pros:

Massive burst, can kill someone instantly
You can actually help your team with your shield despite it having really bad scaling.

Cons:

You really don't have much health, this build is REALLY like "glass cannon"
No escape, this is really just a problem with lux her self.

Lux is very hard to play at the start, the skill shots can be hard to aim if you haven't played skill shot champs before. One thing you should know is that your Q can snare two targets, thus making it possible to snare through minions(minions? haven't you killed all of those already?) which makes it a bit easier to get them if they stay behind their minions.
In a team fight you should ALWAYS open with your ult as soon as they line up so you can hit a lot of them with it, this will allow your them to have an easy go at low health targets. The best part of this is you can then finish with your ult to catch any runners.